As electric, electronic, communication and computer industries have been developed rapidly, high-capacity batteries have been increasingly in demand. In respond to such demand, lithium metal secondary batteries using lithium metal or lithium alloy as a negative electrode having high energy density have been given many attentions.
A lithium metal secondary battery refers to a secondary battery using lithium metal or lithium alloy as a negative electrode. Lithium metal has a low density of 0.54 g/cm3 and a significantly low standard reduction potential of −3.045 V (SHE: based on the standard hydrogen electrode), and thus has been spotlighted best as an electrode material for a high-energy density battery.
When using lithium metal as a negative electrode, it reacts with impurities, such as an electrolyte, water or an organic solvent, or lithium salt, thereby forming a solid electrolyte interphase (SEI) layer. Such a solid electrolyte interphase layer causes a local difference in current density, and thus accelerates formation of dendrite due to lithium metal upon charging. In addition, such dendrite grows gradually during charging and discharging to cause an internal short-circuit between a positive electrode and a negative electrode. Further, dendrite has a mechanically weak part like a bottle neck, and thus forms dead lithium which loses electric contact with a current collector during discharging, resulting in a decrease in battery capacity and degradation of cycle life and battery stability.
To solve the above-mentioned problems, a method for forming a protective film on the surface of lithium metal has been suggested. Japanese Patent Publication No. 3417054 discloses a method for forming a protective film on the surface of a negative electrode by using HF generated in an electrolyte after incorporating a lithium salt for generating HF and water to the electrolyte. However, the protective film formed by the above-mentioned method causes adhesion of various compounds to the lithium metal surface depending on the ingredients of organic solvent or lithium salt in the electrolyte, add thus provides low purity of LiF ingredient in the protective layer and shows a small thickness of LiF layer. In addition, charging/discharging processes are essentially included in forming the protective layer. Such a conventional method for forming a protective layer is problematic in that it includes a complicated process, provides low productivity and has a difficulty in forming a protective film.